Previous work from this laboratory has shown that the hemochorial placentas of the rabbit and the guinea pig do not strongly discriminate molecular size. It is also known that the epitheliochorial placenta of the sheep so strongly discriminates size that the placenta is essentially opaque to hydrophilic materials of greater than 200 molecular weight. It is the purpose of this proposal to prove our hypothesis that the permeability characteristics of the human placenta will be like those of the other hemochorial placentas and not like those of the placenta of the sheep. Nontoxic, nonradioactive, and metabolically inert tracers will be given i.v. to pregnant women shortly before their scheduled elective cesarean sections. At the time of birth, fetal tracer contents will be estimated from fetal plasma concentrations and an approximation of the distribution volume of the tracer and/or from the integral collection of fetal urine until all tracer has been excreted. The timing of the injections will be such that at birth fetal plasma concentrations will be less than 10% of the final maternal plasma concentrations. The transplacental concentration difference is therefore essentially equal to the maternal plasma concentration. The quantitative results from this study should be of help in calculating fetal drug exposures when hydrophilic drugs are present in maternal plasma in known concentrations. They should also allow the construction of a more accurate theoretic model for prenatal water transfer, and thus help elucidate the pathophysiology of hydrops fetalis.